1. Field of the Invention
This invention relates to a method and apparatus for developing electrostatic images, and more particularly, to developing the images with small toner particles safely.
2. Description of the Prior Art
Various development systems are known in the art of developing electrostatic images including cascade, powder cloud, liquid, magnetic brush, and the like, each of which employs electroscopic working particles that are preferably charged to a polarity. The charged particles are presented to the latent images to develop them. One common drawback of many of the heretofore-mentioned development systems is dust, which further requires mechanisms and systems to contain the dust.
In U.S. Pat. No. 2,690,394 to Carlson issued Sep. 28, 1954, there is described a system of electrostatic imaging which utilizes an atomizer to produce a spray of droplets to a solvent. The droplets of solvent are charged by an electrode and then are drawn by means of a suction pump over a recording surface including an electrostatic image. Droplets of solvent are attracted by the oppositely charged image and deposited on the recording surface. The solvent is then transferred from the recording member to a sheet of paper and the paper then passes over a dye-coated roller so that some of the dye is dissolved onto the paper to produce an image. Solvents proposed include high boiling point alcohols, cellosolve, toluene, cyclohexanol acetate and alcohol-water mixtures. It is also suggested that mixtures of solvent with an ink or dye can be atomized and also that the solvent can contain suspended pigments and binders. There is no disclosure, however, of a liquid system where the liquid is composed primarily of water.
It has been disclosed in Heine-Geldern et al., U.S. Pat. No. 3,795,443, issued Mar. 5, 1974, that some liquids are not always characterized as being electrically resistive when generated into a fine mist in the vicinity of an electrostatic charge pattern and will deposit selectively onto an electrostatic image. By chemically coloring the liquid, as by dyes and pigments the selective deposition produces an image reproduction which optionally can subsequently be transferred from or fixed directly on the image bearing surface. The development of this means is effected without using a carrier. Such a system experiences difficulty in developing solid areas without the benefit of external control means such as a development electrode.
Such methods as disclosed by Heine-Geldern et al., are characterized by inadequate development of the electrostatic image by mists such as ultrasonically generated liquid mists. Typically, only small amounts of colorant, be it dye stuff or pigment, deposit onto an electrostatic image on the recording member, be it a photoconductor or dielectric, in a reasonable time, with respect to a practical device for office or industrial reproduction. Thus, commercialization of such an apparatus, as disclosed in Heine-Geldern et al. and Carlson is seen as difficult with regard to the excessively long time that is required for developing of the latent image, not withstanding the low quantity of coloring material which as a result is deposited onto the recording member during such time.
Indoor air quality in offices and other areas where electrostatic image development is used can be seriously affected by hydrocarbon solvents which characterize many liquid development systems and dust which characterizes many powder development systems. There is considerable advantage if an electrostatic image development system could be provided which uses water as the main carrier rather than a hydrocarbon solvent. Such a system is disclosed in an International Application published under the PCT as WO 95/10800. All of the heretofore-mentioned references are incorporated herein by reference to the extent necessary to practice the present invention.
However, there is still a need for an electrostatic image development system that can accommodate custom color on demand and to provide safety from hazards associated with the manufacture, packaging, distribution and machine installation of ultra small toners sized less than 5.mu..